1. Field of the Invention
The present invention relates to a method for preparing a 5,5'-bi-1H-tetrazole diammonium salt that is useful as a lowly toxic and easy-to-handle gas-generating agent for air bags and as a high molecular foaming agent.
2. Description of the Prior Art
A 5,5'-bi-1H-tetrazole (BHT) and its salts have a chemical structure represented by the following formula ##STR1## wherein X is a hydrogen atom or a pair of cations.
There have been known the following four methods of synthesis.
Prior Art 1, Chemical Abstracts Vol. 31, 4985
This literature teaches the synthesis of BHT by the reaction expressed by the following formula (2), ##STR2## Prior Art 2, Friederich DE 952,811, 1956
This literature discloses a method for the preparation of a bistetrazole by reacting a mole of sodium azide or hydrogen azide with 2 moles of sodium cyanide or hydrogen cyanide in the presence of a small amount of copper salt, and teaches, in the working examples, the recovery of the bistetrazole (BHT) in the form of a bisodium salt (BHT.2Na) by condensing the solution after the reaction.
This reaction is expressed by the following formula (3), ##STR3## Prior Art 3, Friederich DE 952,811, 1956, U.S. Pat. No. 2,710,297, 1955
This is the same literature as the one quoted above, and teaches the synthesis of the BHT.2Na by the reaction in accordance with the following formula (4), ##STR4## Prior Art 4, F. Einberg, J. Org. Chem., 29, (1954) 2021
This literature discloses the synthesis of a 5,5'-bi-1H-tetrazole diammonium salt (BHT.2NH.sub.3) by the reaction expressed by the following formula (5), ##STR5##
The prior art 1 teaches the process for obtaining the 5,5'-bi-1H-tetrazole by separating a 5,5'-bi-1H-tetrazole silver salt through the step of forming an azide of an oxalic acid dihydrazide which is a reaction product of a dicyan and a hydrated hydrazine, reacting the 5,5'-bi-1H-tetrazole silver salt with hydrogen sulfide to move it in the form of a silver sulfide. However, this process involves complex steps and requires the use of expensive silver salt and hydrogen sulfide which is toxic.
The prior art 2 teaches the process for synthesizing and isolating the 5,5'-bi-1H-tetrazole disodium salt by using hydrogen cyanide and sodium azide as starting materials. Since the 5,5'-bi-1H-tetrazole disodium salt is soluble in water, it becomes necessary to conduct the step of after-treatment such as condensation for isolating the 5,5'-bi-1H-tetrazole disodium salt from an aqueous solution thereof. Though there has been described that the 5,5'-bi-1H-tetrazole disodium salt is isolated from the aqueous solution through the after-treatment such as condensation, there is no description related to the yields and properties of the isolated compound. The present inventors have conducted trace experiment of Examples of the prior art 2 to find that the yield was as very low as about 30%.
The prior art 3 synthesizes the 5,5'-bi-1H-tetrazole disodium salt by using sodium cyanide, sodium azide and manganese dioxide as an oxidizing agent. However, use of manganese dioxide as an oxidizing agent requires cumbersome after-treatment for removing manganese dioxide.
The prior art 4 uses sodium dinitroacetonitrile, sodium azide and ammonium chloride as starting materials to isolate the 5,5'-bi-1H-tetrazole diammonium salt. However, the reaction time is long, the yield is low and, besides, the sodium dinitroacetonitrile which is the starting material is not easily available.
When the sodium cyanide or hydrogen cyanide is used as a starting material, furthermore, an oxidizing agent is necessary. When a metal salt is used as the oxidizing agent, the reaction intermediate product, i.e., 5,5'-bi-1H-tetrazole metal salt is isolated from the reaction solution and is decomposed arousing, however, a problem of lengthy and complex reaction operation. Besides, use of a heavy metal in the reaction system requires the after-treatment for its removal, which is a serious problem.
In this sense, the method which uses hydrogen peroxide as an oxidizing agent is advantageous but still offers a low yield due to the side reaction of cyan and leaves much room for improvement.
In order to isolate the 5,5'-bi-1H-tetrazole or the 5,5'-bi-1H-tetrazole disodium salt soluble in water, furthermore, the operation such as condensation is required, causing an increase in the number of the steps. Besides, the yield is not satisfactory.